1. Field of the Invention
This invention relates to fusion processes for producing sheet glass and, in particular, to fusion processes which employ a refractory forming body disposed within an enclosure.
2. Technical Background
The fusion process is one of the basic techniques used in the glass making art to produce sheet glass. Compared to other processes known in the art, e.g., the float and slot draw processes, the fusion process produces glass sheets whose surfaces have superior flatness and smoothness without the need for post-forming surface grinding or polishing. As a result, the fusion process has become of particular importance in the production of the glass substrates used in the manufacture of liquid crystal displays (LCDs).
In an exemplary fusion downdraw process, once steady state operation has been achieved, molten glass overflows the top of a trough in a forming body on both sides to form two streams of molten glass that flow downward and inward along outer converging surfaces of the forming body. The two streams meet at the bottom or root of the forming body where they fuse together into a single glass ribbon. The glass ribbon is then fed to drawing equipment that draws the glass ribbon from the root of the forming body. The rate at which the ribbon is drawn influences the final thickness of the ribbon.
The ability to draw molten glass from the forming body in a continuous ribbon with flat, smooth surfaces requires precise temperature control of the molten glass as the glass is supplied to and drawn from the forming body. Such temperature control is typically provided by heating elements disposed in a plenum formed by an enclosure surrounding the forming body Inner enclosure walls positioned between the heating elements and the forming body are typically good conductors of heat in order to effectively transfer heat energy from the heating elements to the forming body. They also serve to even out the heat energy and eliminate hot spots. Nevertheless, the thermal conductivity of the inner wall also serves to conduct heat upward so that power changes in the lower heating element(s) can result in a temperature increase at the top of the enclosure, effectively coupling the upper temperature to changes in the lower enclosure. Thus, it becomes difficult to control the temperature at the root without affecting glass temperature at the top of the forming body.